The present invention pertains to a vehicular automatic transmission that drives a vehicle to move by converting and transmitting the output from the engine, and more specifically, to an apparatus that controls the automatic transmission for a vehicle that traveling while towing a trailer or the like.
As a vehicular automatic transmission, a mechanism that combines a torque converter with a transmission mechanism (such as a gear-type transmission mechanism) is applied to practical use in the conventional art. In this automatic transmission, shift control (also known as speed ratio control or ratio control) (1), in which shifting is automatically carried out in accordance with the degree of engine (drive source) throttle opening and the vehicular speed, such that the vehicle starts in a low speed range, and gradual upward shifting is thereafter performed as the driving speed increases until the highest speed range is reached when the vehicle is cruising at a high speed, is generally performed.
Sometimes, a trailer, camper or the like is attached to and towed behind a vehicle having this type of automatic transmission (the unit being towed in this fashion shall be generally referred to as a xe2x80x98trailer, etc.xe2x80x99). When a trailer, etc. is being towed in this fashion, because the towing load is added to the vehicle, when a load is being towed a type of transmission control different from that performed during normal driving must be performed.
Accordingly, Japanese Patent Laid-Open No. 61-135881 discloses a transmission control apparatus that includes a detector comprising a switch that detects whether or not a trailer is hooked up to a hitch used for trailer towing, and where it is detected by this detector that a trailer is hooked up to the trailer towing hitch, the a gear position of the transmission is controlled to a prescribed low-speed range position. In addition, Japanese Patent Laid-Open No. 8-164832 discloses a limit switch that is mounted to a coupler that connects the trailer [to the vehicle] and detects the towing weight, such that the towing weight detected by this limit switch is used to control the force applied by a retarder or a secondary brake.
Incidentally, where a trailer, etc. is being towed at high speed on a flat road (referred to below as xe2x80x98high-speed cruisingxe2x80x99), the driving load is higher than when no trailer, etc. is being towed. As a result, the problem occurs that high-load and high-RPM driving occurs while the speed ratio of the torque converter is less than 1.0, thereby increasing the heat generated by the torque converter or the like, and consequently the transmission fluid in the transmission can easily become hot. Moreover, due to the changes in load that occur during high-load driving, the problems arise that (1) the internal temperature of the transmission itself increases and (2) the feeling of enjoyment during driving decreases due to frequent shifting of speed ranges upward or downward and frequent changes in the ON/OFF status of the torque converter lockup clutch.
An object of the present invention is to provide an automatic transmission control apparatus that enables comfortable high-speed cruising while preventing the transmission fluid temperature from increasing even when a trailer, etc. is being towed during such high-speed cruising.
In the present invention, the vehicular automatic transmission comprises a torque converter that is connected to a drive source (such as the engine E in the embodiment hereof) and an automatic transmission mechanism that is connected to the output side of this torque converter, and the drive power from the drive source for which speed range shifting is performed via the torque converter and the automatic transmission mechanism is transmitted to the wheels and propels the vehicle. The control apparatus for this automatic transmission mechanism has towing state presumption means that presumes based on the vehicle""s driving conditions that the vehicle is in a towing state, and a lockup engagement increase mechanism that increases the degree of engagement of the lockup clutch of the torque converter when it is presumed via the towing state presumption means that the vehicle is in a towing state.
The towing state presumption means can be constructed such that it determines whether or not the vehicle is in a towing state based on the driving load on the vehicle, the shift frequency, the frequency of operation of the lockup clutch, changes in vehicular speed, or changes in the throttle opening amount, and the means employed in this embodiment that calculates the towing mode determination counter integrated value STRCNT, for example, is equivalent to such means.
According to the vehicular automatic transmission control apparatus pertaining to the present invention having the construction described above, because the degree of engagement of the lockup clutch of the torque converter is increased (i.e., it is engaged tightly) where it is presumed by the towing state presumption means based on the additional towing load that the vehicle is in a towing state when, for example, the vehicle is being driven at high-speed cruising while towing a trailer, the amount of slippage in the torque converter is reduced, such that the amount of heat generated by the torque converter is reduced, and an increase in the temperature of the transmission fluid is inhibited.
It is also acceptable if the control apparatus of the present invention is constructed such that (1) shifting to the highest speed range is prohibited when it is presumed by the towing state presumption means that the vehicle has entered a towing state, (2) where the current speed range is the highest speed range, control is performed to shift down to a lower speed range, or (3) the degree of engagement of the lockup clutch is increased by the lockup engagement increase mechanism when it is presumed by the towing state presumption means that the vehicle has entered a towing state even after a shift down is performed.
Because setting of [the transmission] to the highest speed range is prohibited and the highest speed range in which the vehicle can be driven is the speed range at least one speed range lower than the highest speed range when it is presumed by the towing state presumption means that the vehicle has entered a towing state, the control apparatus having the construction described above reduces the drive torque of the torque converter and the driving load thereon, thereby reducing the amount of heat generated by the torque converter. Where the vehicle subsequently remains in the towing state, the degree of engagement of the lockup clutch of the torque converter is increased, the amount of slippage in the torque converter is limited, and the amount of heat generated thereby is accordingly reduced. As a result, the amount of heat generated in the transmission while in the towing state is reduced in stages, and overheating of the transmission fluid can be effectively prevented.
Where shifting down is to be performed based on the presumption by the towing state presumption means that the vehicle is in a towing state while being driven at the highest speed range, it is preferred that the shift down be carried out only after the accelerator pedal is pressed or a shift down instruction is issued. In this way, the shift down can be performed without a lurching feeling.
Furthermore, when the vehicle is stopped while it is presumed by the towing state presumption means that the vehicle is in a towing state and driving is thereafter resumed, it is preferred that the presumption regarding the vehicle""s towing state be carried out based on a decrease in the transmission fluid temperature during the period that the vehicle is stopped. This enables appropriate control to be performed after the vehicle has been stopped temporarily.